Chapter 6 Conditionals

Learning Java through Alice © Daly and Wrigley Objectives List relational operators. List logical operators. Use the hierarchy of operators chart to properly construct if/else statements. Construct switch statements. Use nested if statements. 2

Learning Java through Alice © Daly and Wrigley Decisions True and false values are also known as Boolean values, named after the 19th century English mathematician George Boole. 3

Learning Java through Alice © Daly and Wrigley Relational Operators A Boolean test compares primitives, constants, and variables and or objects using the following relational operators: OperatorMeaningExample ==Equal tox == 3 ! =Not equal tox != 3 <Less thanx < 3 >Greater thanx > 3 <=Less than or equal tox <= 3 >=Greater than or equal tox >= 3 Note: These relational operators must be typed exactly as above. You can’t type a space between the two equal signs and you can’t put =< to mean less than or equal. 4

Learning Java through Alice © Daly and Wrigley Alice Example of an if statement : 5

Learning Java through Alice © Daly and Wrigley Logical Operators Logical OperatorDescription &&Short-curcuiting AND ||Short-circuiting OR &AND |OR !NOT ^Exclusive OR 6

Learning Java through Alice © Daly and Wrigley If Statements The syntax contains the keyword if, followed by a condition (boolean test) in parenthesis, followed by either a single statement or a block statement to execute if the test is true. An optional else keyword provides the alternative statement to execute if the test is false. if (condition) { statements to do when conditional is true } else { statements to do when conditional is false } 7

Learning Java through Alice © Daly and Wrigley If Statements Examples if (x < y) { System.out.println ( "x is smaller than y" ); } else { System.out.println ("x is not smaller than y" ); } 8

Learning Java through Alice © Daly and Wrigley If Statements Examples Continued… if ( guess == correctAnswer ) { score = score + 10; System.out.println ( "You are right. You get 10 points."); } else { score = score - 5; System.out.println ( "Sorry, that is wrong. You lose 5 points."); } 9

Learning Java through Alice © Daly and Wrigley Else If if (x < 0) { System.out.println ( "X is negative." ); } else if (x == 0) { System.out.println ( "X is equal to 0." ); } else { System.out.println ( "X is positive." ); } 10

Learning Java through Alice © Daly and Wrigley Using Logical Operators The following only prints if x is less than y and x is less than z. The && means if x is NOT less than y it won't even bother to test to see if x is less than z. if ( x < y && x < z) { System.out.println ("x is less than both y and z" ); } The following calculates grossPay if either condition is true. The || means that if weeklyHours is less 40, it won't bother to test to see if employeeType is 'P'. if (weeklyHours < 40 || employeeType == 'P' ) { grossPay = weeklyHours * hourlyRate; } 11

Learning Java through Alice © Daly and Wrigley Switch Statements Done with if statements:Done with switch statements: if (grade == 'A') { System.out.println("Great!"); } else if (grade == 'B') { System.out.println("Good!"); } else if (grade == 'C') { System.out.println("Nice."); } else { System.out.println("Not Good."); } switch (grade) { case 'A': System.out.println("Great!"); break; case 'B': System.out.println("Good!"); break; case 'C': System.out.println("Nice."); break; default: System.out.println("Not Good."); } 12

Learning Java through Alice © Daly and Wrigley Complex If Statements vs. Switch Complex If Statements:Switch Statements: if (month == 2) { days =28; } else if ((month == 4) || (month == 6) || (month == 9) || (month == 11)) { days=30; } else { days=31; } switch (month) { case 2: days = 28; break; case 4: case 6: case 9: case 11: days = 30; break; default: days = 31; } 13

Learning Java through Alice © Daly and Wrigley Comparing Strings using if/else 14

Learning Java through Alice © Daly and Wrigley Comparing Strings using the Switch 15